1. Field of the Invention
The present invention relates to an automatic calibration method for use in an electronic compass, and more particularly to an automatic calibration method for use in an electronic compass, which automatically calculates and corrects offset and scale values of a geomagnetic signal by detecting one rotation of a geomagnetic axis during a predetermined period of time using a timer, performs automatic calibration capable of calculating offset and scale values of a geomagnetic signal without receiving additional entry data from a user, and quickly copes with environmental variation.
2. Description of the Related Art
In recent times, there have been developed small-sized and low-priced geomagnetic sensor modules and electronic compasses. With the increasing development of the MEMS (Micro-Electro Mechanical Systems) technology, chip-sized geomagnetic sensor modules have been newly developed and used for a variety of applications. There is a need for the geomagnetic sensor to control offset and amplitude (scale) values of an x-axis to be equal to those of a y-axis in the case of using two-axis (x-axis and y-axis) signals. In the case of using three-axis signals, there is a need for the geomagnetic sensor to use calibration circuits or programs for x-axis, y-axis, and z-axis.
FIG. 1 is a block diagram illustrating a conventional electronic compass.
Referring to FIG. 1, the conventional electronic compass includes a geomagnetic sensor 11 for detecting a geomagnetic azimuth angle; an analog processor 12 for amplifying a detection signal of the geomagnetic sensor 11, and filtering the amplified detection signal to remove noise; an analog/digital (A/D) converter 13 for converting a geomagnetic signal received from the analog processor 12 into a digital signal using an A/D conversion process; and a digital processor 14 for calculating a geomagnetic azimuth angle on the basis of the digital signal received from the A/D converter 13, and performing a calibration process to calculate offset and scale values.
In this case, the offset value is an intermediate voltage of an AC (Alternating Current) waveform, and is denoted by the following Equation 1. The scale value is associated with the width between a maximum value and a minimum value of the AC waveform, and is denoted by the following Equation 2. Therefore, deviation of the azimuth angle can be corrected or calibrated using the above offset and scale values.
The geomagnetic sensor 11 is a prescribed sensor for detecting and measuring the earth's magnetic field intensity, and includes x-axis, y-axis, and z-axis sensors arranged at right angles to each other.
However, the conventional electronic compass has a disadvantage in that it must receive an entry signal from a user to carry out a desired calibration process, such that it cannot perform the calibration process without receiving the user's entry signal. In addition, the output signal of the geomagnetic sensor 11 abruptly varies with peripheral environments, such that offset and amplitude (scale) values of the output signal of the conventional electronic compass may also abruptly vary with the peripheral environments.
Therefore, provided that offset and amplitude values of the output signal of the electronic compass are unexpectedly changed in the case of carrying out the calibration process only using the user's entry signal, changed offset and amplitude values are real-time reflected in a resultant geomagnetic signal of the electronic compass, resulting in difficulty in calculating a correct azimuth angle.
A method for correcting or calibrating the offset and amplitude (scale) values created in the aforementioned conventional electronic compass will hereinafter be described with reference to FIG. 2.
FIG. 2 is a flow chart illustrating a calibration method for use in the conventional electronic compass.
Referring to FIG. 2, a controller serving as a calibration controller receives a calibration signal from an external switch or a host processor to carry out a calibration method of the conventional electronic compass at step S21. The controller operates a sensor at step S22, and receives data detected by the sensor at step S23. In this way, the conventional electronic compass can start the above calibration process after receiving a user's request signal. If the calibration process has started, the electronic compass drives the geomagnetic sensor to generate an analog signal therefrom.
The analog signal generated from the geomagnetic sensor is converted into a digital signal according to an A/D conversion process. The output signal of the geomagnetic sensor positioned horizontally to the horizon is represented by sine or cosine wave signals associated with the azimuth angle, as shown in FIG. 3.
Thereafter, the controller calculates maximum and minimum values of the output signal of the geomagnetic sensor using the entry data at step 524, and determines whether 1-closed loop equal to one rotation is provided at step S525. If it is determined that the 1-closed loop equal to one rotation has been provided, the controller calculates offset and scale values of the geomagnetic signal of the geomagnetic sensor on the basis of the entry data at step S26, and stores the calculated offset and scale values at step S27. In other words, if the geomagnetic sensor is rotated at least one time, the controller finishes collecting data of the geomagnetic sensor to carry out the above calibration process.
If the geometric sensor is rotated more than one time, the controller stores calibration data in a nonvolatile memory such as an EEPROM or flash memory. Data stored in the memory includes minimum and maximum values of individual axes or scale and offset values calculated using the minimum and maximum values of the individual axes. The process for calculating the scale and offset values indicates a calibration process of the present invention. In this case, scale and offset values associated with individual axes are represented by the following Equations 1 and 2, respectively:Scale=C/(Max−Min), where C is an arbitrary constant  [Equation 1]                    Offset        =                              (                          Max              +              Min                        )                    2                                    [                  Equation          ⁢                                           ⁢          2                ]            
However, the above-identified conventional calibration method has a disadvantage in that it requires a user's data entry process to carry out a calibration process capable of calculating offset and scale values of individual axes of the geomagnetic sensor. Furthermore, provided that peripheral environmental conditions are unexpectedly changed during an operation time of the electronic compass, the aforementioned conventional calibration method cannot update calibration data according to the changed peripheral environmental conditions.